refactor: remove Redis benchmark system

Remove the Redis-backed performance benchmarking in favor of the new
lightweight file-based tracing system which provides better debugging
capabilities for local development.

- Delete src/core/benchmarks.py
- Remove ENABLE_BENCHMARKS, REDIS_BENCHMARK_DB, redis_url from config
- Update memory_cache comment (now uses DB 1)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/memory_cache.py

@@ -6,7 +6,7 @@ Provides short-term memory storage with TTL:
 - Recent entities mentioned in conversation
 - User-scoped with conversation isolation
 
-Uses Redis DB 2 (separate from benchmarks in DB 1).
+Uses Redis DB 1.
 """
 import json
 from typing import Any